Polymer based composites often serve as a substitute for traditional industrial materials; these composites are relatively easy to fabricate and are becoming ubiquitous in the materials industry. An increased use of polymer based composites designed for specific uses has stimulated theoretical efforts oriented at describing and modeling their properties. Models often rely on information concerning boundary conditions for continuity of displacement and stress between the constituents of a composite and investigators generally assume rigid bonding as described by L. Piche (the applicant) and A. Hamel, in Polym. Compos. 7, 355, 1986; 8,22,1987. However, when polymers are involved, this description is incomplete and viscoelastic behavior must be accounted for. Bonding is never perfectly stiff (good bonding), nor perfectly soft (bad bonding); instead, one should make reference to a quantitative criterion related to materials properties.
Recently, a great deal of progress has been made in understanding the physical structure and properties of polymer surfaces and interfaces. On one hand, efforts have been devoted to theoretical modeling either through analytical work or numerical simulations. On the other hand, new techniques provide density profiles with finer spatial resolution, such as Scanning Electron Microscopy (SEM, resolution 100 nm), Forward Recoil Spectroscopy (FRES, resolution 80 nm), Dynamic Secondary Mass Spectrometry (DSIMS, resolution 10 nm), Nuclear Reaction Analysis (NRA, resolution 10 nm), Small-Angle Neutron-Scattering (SANS, resolution 10 nm), Neutron Reflectivity (NR, resolution 1 nm), Transmission Electron Microscopy (TEM, resolution 1 nm), X-ray Photoelectron Spectroscopy (XPS, resolution 0.1 nm), in addition to more indirect means like Infrared Spectroscopy and Attenuated Total Reflectance Infrared Spectroscopy (ATR-IR, resolution 0.1 to 2 .mu.m), or thermodynamic and hydrodynamic methods. Tools have also been developed that probe interfacial forces and complement traditional measurements. Newer methods include Scanning Tunneling Microscopy (STM), Atomic Force Microscopy (AFM), the quartz microbalance, and most relevant, the Surface Force Apparatus (SFA) that measures forces between surfaces as function of separation (resolution 0.1 nm) and between surfaces that confine fluids. Finally, techniques have been developed to study mobility near interfaces and to assess the dynamic properties of confined molecular systems.
These new methods, however, are not directly applicable to the characterization of adhesion in real structures because the interfaces are not accessible for measurements and because practical adhesion--for example, as measured in peel tests--relates to complex nonlinear and non reversible processes that involve other materials characteristics. Notwithstanding, interfacial chemistry and interfacial interactions are essential factors in the making and breaking of bonds. Therefore, a nondestructive technique that correlates both to interfacial properties and irreversible mechanical behavior would be of great benefit.
Amongst the different approaches that have been investigated, namely nuclear magnetic resonance, acoustic emission, optical and thermal methods, ultrasonics have emerged as the most promising. Ultrasonic techniques have become more dependable for the detection and identification of defects due to lack of adhesive, delamination, porosity or surface roughness. However, it is only rather recently, that a significant amount of research has been devoted to the problem of interface adhesion per se.
It is an object of the invention, to provide a more reliable robust method for characterizing interfacial adhesion in a multilayer structure.
The invention provides an ultrasonic method that characterizes interfacial adhesion in terms of physical parameters. The method is based on analyzing the interferometric features associated with the reverberation of ultrasound in a multilayered system. The nature and the origin of interfacial adhesion is more clearly identified and a numerical scheme is presented for modeling the propagation of ultrasound in layered media; the invention provides a method for determining a simple materials constant which is an expression of interfacial adhesion in the form of a specific stiffness for the interfacial layer.
In accordance with the invention, a method is provided for characterizing the interfacial adhesion in a multilayer composite having at least three layers. The method comprises the steps of: irradiating the multilayer composite with a pulsed signal, each pulse having a width of about less than or equal to 100 ns to obtain a first signal characteristic of the interfacial adhesion between the multilayers; detecting a signal from the irradiated object; storing amplitude and phase information corresponding to the detected signal; convening the stored information from the time domain to the frequency domain to obtain a frequency spectrum related to the stored information; and, comparing the frequency spectrum with a spectrum derived from a model of a multilayer composite that includes at least two additional layers which model the behaviour of interface between the composite layers.
In accordance with the invention there is further provided, a method of characterizing the interfacial adhesion in a multilayer composite comprising the steps of:
a) ultrasonically irradiating the multilayer composite having at least 3 layers to obtain a first signal characteristic of the interfacial adhesion between the multilayers; PA1 b) modeling the multilayer composite with a model that includes at least two additional layers which model the behaviour of the interface between the composite layers in such a manner as to obtain plurality of spectra, each spectrum being characteristic of interfacial adhesion between the modeled multilayer composite; and, PA1 c) comparing, in the frequency domain, a first signal characteristic of the interfacial adhesion between the multilayers with a plurality of the spectra obtained in step (b) to obtain a spectrum which most closely matches the spectrum of the first signal.
Advantageously, the invention provides a method of ultrasonically irradiating a composite, and for obtaining a measure in the form of a quantifiable index of the adhesion of the bond between the layers of the composite. The method not only allows a bond to be tested, but as well, it can be used as a tool so that bonds will meet specific desired levels of adhesion.